Change library search methods to return Optional instead of ErrorOr.

These methods weren't really throwing errors.  The only error used
was that a file could not be found, which isn't really an error at all
as we are searching paths and libraries for a file.  All of the callers
also ignored errors and just used the returned path if one was available.

Changing to return Optional<StringRef> as that actually reflects what
we are trying to do here: optionally find a given path.

llvm-svn: 264979